The tornado that plowed a wide swath of death and destruction through Joplin, Mo., on Sunday unleashed winds of up to 198 miles per hour, federal forecasters said yesterday.

The National Oceanic and Atmospheric Administration's preliminary analysis ranks the twister as an F4, the second-highest rating on the five-point scale used to classify tornadoes.

Agency officials said the Joplin storm, at times three-quarters of a mile wide, was the deadliest single tornado to hit the United States since 1953. At least 116 people died and 500 were injured by the storm. The numbers are expected to climb as aid workers comb through the wreckage left behind.

"This is the ninth-deadliest tornado year on record so far," said Jack Hayes, director of NOAA's National Weather Service. "More than 450 people have been killed. ... With so many fatalities this year, I think we have to ask ourselves the tough questions now. Why is this happening?"

But experts said there is no easy explanation for the ferocious intensity of this year's tornado season.

"Certainly, I think you could say we have a high-impact season," Hayes said. "I think we have more F4s and F5s than in past years -- but for us to say this is climate change, I don't know that we have the evidence that we can say that definitively."

Unusually warm surface water in the Gulf of Mexico -- about 2 degrees Fahrenheit warmer than normal -- may be a factor, he said.

A 'remarkable pattern' shifts killer storms eastward

"When you have a long, prolonged stretch of warm humid air coming in from the Gulf of Mexico, you are going to set up conditions that are ripe," Hayes said. "And then, when you have these slow-moving fronts and an injection of cooler air, it collides and triggers this instability."

That collision between very warm, humid air at low levels of the atmosphere and cool air at higher levels creates the upward vertical winds within a thunderstorm that sometimes turn into a tornado, said Thomas Schwein, deputy director of the National Weather Service's Central Region, which includes Missouri.

This year, the jet stream has been dipping farther south than normal, allowing it ready access to that warm Gulf of Mexico air. "It's been a remarkable pattern from a day-to-day weather perspective," Schwein said. That has also shifted this year's tornadoes farther to the east, into more densely populated sections of the country, which may account in part for the high number of fatalities.

Another expert suggested a potential influence from the current La Niña weather pattern, in place since last summer.

"We have been coming out of a La Niña," said Russell Schneider, director of NOAA's Storm Prediction Center. "There is some emerging research that links more active severe-weather seasons -- particularly months like April -- to emerging within a La Niña." Such a pattern was observed in 1965 and 1974, which Schneider called "major springtime severe-weather outbreak years."

NOAA's storm survey crews hope to issue a final storm strength rating today for the Joplin tornado. The agency sent additional crews to Missouri yesterday to examine the pattern of destruction left by the twister, observations NOAA scientists will use to estimate the tornado's power.

Some insurers see climate connection to rising risks

Sunday's tornado also thrusts the insurance industry toward a potential record-breaking year for thunderstorm-related damage. Inland storm claims over the last three years have risen to about $30 billion altogether. That accounts for almost one-third of all the thunderstorm damage going back to 1990, amounting to $97.8 billion, according to the Insurance Information Institute. This year will add billions more onto that tally.

The number and ferocity of those storms have been rising for at least 25 years. It's unclear what's causing the rise, but many insurers are responding by raising rates and reducing policies. Factors behind the damage could include expanded development and rising property values.

But some insurers also believe climate change is playing a part.

The Interinsurance Exchange of the Automobile Club, which collected $2.1 billion in premiums last year, writes homeowners insurance policies in Missouri, Texas and California. Those areas face "catastrophic risk" that could be increased by global warming, the company told California insurance regulators this month.

"The primary risk that climate change may pose would be any potential increase in the frequency or intensity of strong thunderstorms, hurricanes or brushfires," the company said on a climate risk disclosure form that California began requiring insurers to file this year.

"More severe or more frequent events would increase the number and cost of claims and have an impact on the financial health of the company," the Automobile Club wrote. "If the impact was large enough, it could affect our ability to write new or renew business in some catastrophe exposed areas."

About 1,200 tornadoes touch down in the United States annually, but this year is on pace for more. About 1,076 twisters have already been counted -- and that was before the storms in Missouri this week.

The accelerating activity has convinced Allstate Insurance Co. that the change is permanent. Chairman and CEO Thomas Wilson told Wall Street analysts last month that the company now believes it will face about $2 billion in damage annually from thunderstorms -- about four times the amount it paid a decade ago.